Fix inadvertent generation of file during tox test
Fix inadvertent generation of file during tox test

When running py27 or py34 tox tests, a '4321_4321_4321.dat' file is
created and not cleaned up. This is due to a missing mock in the
module_retrieve test.

The 'open' method is now mocked and the file is not created.

Change-Id: I8e2524961b3cb2ac01b65d5ce11d476ad3a81662
Closed-Bug: #1565959

Tesora/tesora-python-troveclient,openstack/python-troveclient,openstack/python-troveclient,Tesora/tesora-python-troveclient